(1) Field of the Invention
The present invention relates to a communication device, and more particularly, to a communication device having a card unit mounted thereon to perform communication control.
(2) Description of the Related Art
With the recent popularization of multimedia services, the development of subscriber transmission systems is advancing rapidly, creating an increasing demand for line terminating devices that achieve user-network interface. A line terminating device is installed in a user's home to carry out high-speed data transmission and includes, for example, a DSU (Digital Service Unit: digital line terminating device) for ISDN, an ONU (Optical Network Unit: optical subscriber line terminating device) used in optical access network for optical transmission, etc.
FIG. 11 shows an external appearance of a conventional line terminating device. A line terminating device 100 is constructed such that a plurality of printed boards are mounted to a shelf-like casing. The casing has guide rails 101 permitting individual printed boards to be inserted and detached, and a backboard 102 interconnecting the printed boards.
Printed boards to be mounted include two board types, a basic board 103 and a signal processing board 104. The basic board 103 is a printed board on which a station interface control section, a power supply, etc. are packaged, and the signal processing board 104 is a printed board for processing signals associated with various services.
A station interface cable 105 extends from the basic board 103 to be connected to a station, and terminal interface cables 106 extend from the respective signal processing boards 104 to be connected to terminals such as personal computers.
A blank board 107, which is mounted to an empty slot for the purpose of protection against EMI (Electro-Magnetic Interference), cuts off electromagnetic noise leaking to the air from the signal processing boards 104 and thereby prevents erroneous operation of electronic circuits on the other signal processing boards 104.
The conventional line terminating device 100 described above is, however, configured such that a space is reserved in advance to accommodate a pre-specified number of signal processing boards, and thus is associated with a problem that the device is large in size and poor in convenience and economy.
Also, since the line terminating device is installed in a home or an office, it is often difficult to find a suitable place to install the device with the conventional structure, and thus there is a strong demand for a smaller-sized device. Further, in the case of the conventional device, the backboard and connectors must be prepared in consideration of spare slots and also blank boards need to be mounted to empty slots, which leads to increased cost of the device.